I will tell you the most common fails i see on newbye tanks, and not so newbye ones.

1- Not having updated gear.
Make sure you at least check for cheap gear on market or even npc.

2- Failing Flash/Overpower for control waves of enemies.
You need to learn the Flash range (its not hard), and to position yourself for overpower if you are war.

3- Delivering hate only on the first/focus target.
You should switch targets and split your enmity combos between, making sure the focused target has enough enmity (depends really on how hard your dps strike), or abusing aoe enmity generation (limited to MP/TP).

4- Not caring about positioning the mobs.
Sometimes your dps mates need to side attack or "backstab" the target, make sure they can do it, and block/parry will only work when you face the targets.

5- Defensive cooldown control.
Make sure you use your skills... but don't do overkill like using them all at once.